Andalusite

Andalusite is a polymorph of kyanite and sillimanite.  These three minerals share the same formula, but are stable at different P/T conditions.

Physical Properties
Chemical formula As2SiO5
Class Nesosilicate
Isolated tetrahedra
Crystal system Orthorhombic
Habit Stubby to elongate square prisms
Individual crystals may be rounded
Massive
Granular
Color Brown to red
Hardness 7.5
Specific gravity 3.18
Cleavage rarely seen, good {110}, poor (100)
Fracture Subconchoidal
Luster Vitreous
Transparency Transparent to translucent
Streak White
Optical Properties
PPL Usually colorless
May be weakly colored and pleochroic
XPL 1st order yellows (pale tan)
May be euhedral with a square outline
and penetration twins or cross pattern
δ 0.010
Twinning Penetration twins or Maltese cross pattern
after Perkins, 345

Andalusite in Hand Sample

Four small subhedral to euhedral andalusite crystals

Andalusite in a rock

Andalusite in Thin Section

Thin Section GigaPans

Andalusite in slate, plane polars

Andalusite in slate, crossed polars

Andalusite, PPL

Andalusite, XPL

Further Reading

Andalusite at webmineral.com

Andalusite at mindat.org

Bonus Content

The song that gets stuck in your friendly author’s head every time she encounters andalusite.